1. Field of the Invention
The present invention relates to an electrodeionization deionized water producing apparatus used in various industries such as the semiconductor manufacturing industry, the pharmaceutical industry, the food industry, power plants, and laboratories, as well as for the manufacture of sugar solutions, juice, wine, and the like.
2. Description of Background Art
As a method of producing deionized water, a method of passing feed water through ion exchange resins has conventionally been known. This method, however, requires regeneration of the ion exchange resins with chemicals when the ion exchange resins have been saturated with ions. To overcome this operational disadvantage, the electrodeionization deionized water production method that does not require regeneration with chemicals has been established and put into practice.
The above conventional electrodeionization deionized water producing apparatus has a basic structure of a deionizing chamber containing an ion exchange resin mixture consisting of an anion exchange resin and a cation exchange resin, packed in a space between a cation exchange membrane and an anion exchange membrane. Feed water is passed through the ion exchange resin mixture and, at the same time, a direct current is applied to the direction vertical to the flow of the water being treated via both the ion exchange membranes to electrically remove ions in the feed water into concentrate water flowing outside the both ion exchange membranes, thereby producing deionized water.
FIG. 5 shows a schematic cross-sectional view of the conventional electrodeionization deionized water producing apparatus, wherein a cation exchange membrane 101 and an anion exchange membrane 102 are alternately disposed apart from each other. A mixed ion exchange resin 103 consisting of a cation exchange resin and anion exchange resin is filled in every other space formed by the cation exchange membrane 101 and the anion exchange membrane 102, thereby forming ion depletion chambers 104. The area formed by the anion exchange membrane 102 and cation exchange membrane 101 respectively adjacent to the ion depletion chambers 104, not packed with the mixed ion exchange resin 103, serves as a concentrate chamber 105 for passing concentrate water as discussed later. A deionizing module 106 is formed by a cation exchange membrane 101, an anion exchange membrane 102, and the mixed ion exchange resin 103 packed between them. The detail is described in FIG. 6.
Specifically, the structure consists of the cation exchange membrane 101 sealingly attached to one side of a frame 107, the mixed ion exchange resin 103 packed inside the frame 107, and the anion exchange membrane 102 sealingly attached to the other side of the frame 107. Since the ion exchange membranes are comparatively soft materials, they may curve when the mixed ion exchange resin 103 is packed inside the frame 107 and the cation exchange membrane and anion exchange membrane 101 and 102 are sealingly attached to the both sides of the frame 107, resulting in an inhomogeneous packed layer. To prevent this, a plurality of ribs 108 are usually formed in the space inside the frame 107. Although not shown in FIG. 6, the frame 107 is provided with an inlet port for water to be treated in the upper part and an outlet port for treated water in the lower part.
FIG. 5 shows a plurality of such deionizing modules 106 arranged with a spacer (not shown) between them. A cathode 109 is provided on one end and an anode 110 is provided on the other end of the arranged deionizing modules 106. The space with the spacer therein forms a concentrate chamber 105. As required, a partition membrane 111 such as a cation exchange membrane, anion exchange membrane, or separating membrane with no ion exchange capabilities is provided outside the concentrate chambers 105 on both sides. The areas contacting the both electrodes 109 and 110 respectively and separated by the partition membrane 111 are respectively a cathode chamber 112 and an anode chamber 113.
Deionized water is produced using such an electrodeionization deionized water producing apparatus in the following manner. A direct current is passed between the cathode 109 and the anode 110. Feed water flows in from a feed water inlet port A, concentrate water flows in from a concentrate water inlet port B, and electrode water flows in from electrode water inlet ports C and D. Water coming in from the feed water inlet port A flows down through each ion depletion chamber 104 in the direction of the arrows shown in solid lines. Impurity ions are removed when the water passes through the packed layer of the mixed ion exchange resin 103. Deionized water is obtained from a deionized water outlet port a. Concentrate water flowing in from the concentrate water inlet port B flows down each concentrate chamber 105 in the direction shown by the dotted line arrows, accepts impurity ions coming in via the both ion exchange membranes, and is discharged from a concentrate water discharge port b as concentrate water in which the impurity ions are concentrated. The electrode water flowing in from the electrode water inlet ports C and D is discharged from the electrode water discharge ports c and d respectively. Since this operation electrically removes impurity ions in the feed water, deionized water can be continuously produced without regenerating the packed ion exchange resins by treatment with chemicals.
However, this conventional electrodeionization deionized water producing apparatus has shortcomings such as a complicated structure and high costs for raw materials, fabrication, and assembly, since the apparatus must be provided with many ion depletion chambers and concentrate chambers which require a number of frames and ion exchange membranes. Specifically, since the conventional electrodeionization deionized water producing apparatus has bead ion exchange resins filled in a deionizing chamber, the transfer of ions when excluding ions adsorbed on the ion exchange resins is slow. This requires a shortened electrophoresis distance for ions to be excluded to obtain the target quality of treated water. Therefore, the thickness of the deionizing chamber is limited to as thin as about 1 to 8 mm. To design the conventional electrodeionization deionized water producing apparatus which can process a practical amount of water with a desired quality while being subject to these restrictions, a structure, in which a number of deionizing chambers in the form of a thin plate are arranged to pass feed water in the direction of longitudinal axis of the deionizing chamber and an electric field is applied in the vertical direction to exclude ions adsorbed, must be applied.
In the conventional electrodeionization deionized water producing apparatus, spherical particles (beads) of ion exchange resin with a diameter of about 0.2 to 0.5 mm made from a styrene-divinyl benzene (DVB) copolymer with a sulfonic acid group (R—SO3−H+) introduced as a cation exchange group and a quaternary ammonium group (R—N+R1R2R3) as an anion exchange group have been used as ion exchange resins to be packed in the deionizing chamber. In these ion exchange resins, current tranfer (or tranfer of ions) in ion exchange resin particles is effected at low-resistance by dint of ion exchange groups uniformly and densely dispersed in the polymer gel, whereas in the interface of ion exchange resin particles, due to a long migration distance of the ions in water and also due to a small contact area between the resin particles having the spherical form, the flow of ions is concentrated on said small contact areas. This phenomenon occurring in the interface of the resin particles leads to hindrance of the transfer of ions, and therefore a major cause of slow discharge of ions outside the system.
In addition, the conventional electrodeionization deionized water producing apparatus has a disadvantage of forming scale such as calcium carbonate and magnesium hydroxide in the concentrate chamber when feed water has a high hardness. Specifically, calcium ions and magnesium ions discharged to the concentrate chamber from the ion depletion chamber via a cation exchange membrane are locally concentrated on the surface of the anion exchange membrane on the opposite side of the concentrate chamber, in which these calcium ions and magnesium ions are mixed with carbonate ions and hydroxide ions discharged to the concentrate chamber via the anion exchange membrane in excess of the solubility product constant, thereby forming scale. If scale is formed, the electric resistance increases in the scaling, resulting in a decreased current flow. To cause the current to flow in a quantity equivalent to that observed when there is no scaling, the voltage must be increased, which in turn results in an increased power consumption. In addition, the current density may vary according to the scaled area in the concentrate chamber, which leads to a non-uniform current in the deionizing chamber. If the amount of scaling increases further, the pressure difference of flow increases and the voltage is further augmented. The current decreases when the voltage exceeds the maximum voltage for the apparatus. In this instance, the current necessary for removing (excluding) ions does not flow, thereby giving rise to a decrease in the quality of treated water. In addition, grown scaling invades the inside of the ion exchange membrane and ultimately breaks the ion exchange membrane.
To prevent the scale formation, in the conventional electrodeionization deionized water producing apparatus hardness components such as calcium ions and magnesium ions are removed from feed water before introducing it to the electrodeionization deionized water producing apparatus. Specifically, the conventional electrodeionization deionized water producing apparatus required softening by replacing the hard components with sodium ions by causing the feed water to pass through a Na-type cation exchange resin layer or primary desalting treatment using a reverse osmotic membrane or ion exchange membranes.
Accordingly, an object of the present invention is to provide an electrodeionization deionized water producing apparatus ensuring easy removal of ions adsorbed on the ion exchange resins by accelerating the movement of adsorbed ionic impurities, capable of decreasing the costs of raw materials, fabrication, and assembly by simplifying the structure of the apparatus, and entirely free from scale formation such as calcium carbonate and magnesium hydroxide, thereby eliminating the necessity of pretreatments such as primary desalting and softening.